标记内部类@objc？

Question

我正在尝试编译一些快速代码

 import Foundation

 let BaseSocketErrorDomain = "com.thing.base_socket";

 @objc internal protocol GATToIPBaseSocketDelegate {
     @objc optional func txReady(_: GATToIPBaseSocket)
     @objc optional func rxData(_: GATToIPBaseSocket, data:Data) -> Bool
     @objc optional func didConnect(_: GATToIPBaseSocket)
     @objc optional func didDisconnect(_: GATToIPBaseSocket, error: Error?)
 }

 @objc internal class GATToIPBaseSocket: NSObject {
     enum SocketError: Error {
         case notConnected
         case noSpaceAvailable
         case sendError
     }

     weak var delegate: GATToIPBaseSocketDelegate?
     private(set) var isConnected: Bool = false

     func openConnection() {
         // nothing yet
     }

 }

导致以下错误

/home/pi/ReWork/GATToIP/GATToIP/BaseSocket.swift:20:2: error: only classes that inherit from NSObject can be declared @objc
@objc internal class GATToIPBaseSocket: NSObject {
~^~~~~

但是，该类非常明显地标记为@objc并且继承自NSObject。谁能告诉我我在这里缺少什么？